Products having helical shapes such as a male rotor or a female rotor of a screw compressor are usually manufactured by, for example, cutting a cylindrical member and then processing a portion of the cylindrical member into a helical shape using a tool specially designed for processing. Such a manufacturing method, however, is disadvantageous in that it requires a wide process margin and a long processing time. A method known to date for reducing a processing time is a method including forming a near-net-shape cast product (replicating the shape to that of a final product by reducing a process margin) and performing a finishing operation on the cast product.
In some cases, a core die used for molding a core having a helical shape and required for casting a near-net-shape object has a portion protruding perpendicularly to the direction in which the core die is removed from the core (for example, an axial direction or radial direction). In such cases, removal of the core die from the core is difficult unless the core die or the core is deformed.
Thus, PTL 1 discloses a method for casting a multiple-threaded component by dividing a core die into, for example, two core-die sections and molding core sections using the respective core-die sections so that the core-die sections can be easily removed from the core sections. Each of the core-die sections includes a screw portion including a crest-diameter portion and a groove gradient portion. The crest-diameter portion is disposed at the crest of a screw thread so that the outer diameter of the screw thread is substantially uniform with respect to the axis serving as a center. The groove gradient portion is disposed in a thread groove and has a predetermined removal gradient with respect to the axis serving as a center. When a rotational force is applied to the core-die sections while the core-die sections are pulled in one axial direction, the crest-diameter portion of each core-die section slides over the groove portion of the corresponding core section to serve as a guide, so that the core-die sections are easily removed from the core sections.